diff --git a/bin/spark-class b/bin/spark-class
index c03946d92e..c49d97ce5c 100755
--- a/bin/spark-class
+++ b/bin/spark-class
@@ -82,13 +82,22 @@ if [ $(command -v "$JAR_CMD") ] ; then
   fi
 fi
 
+LAUNCH_CLASSPATH="$SPARK_ASSEMBLY_JAR"
+
+# Add the launcher build dir to the classpath if requested.
+if [ -n "$SPARK_PREPEND_CLASSES" ]; then
+  LAUNCH_CLASSPATH="$SPARK_HOME/launcher/target/scala-$SPARK_SCALA_VERSION/classes:$LAUNCH_CLASSPATH"
+fi
+
+export _SPARK_ASSEMBLY="$SPARK_ASSEMBLY_JAR"
+
 # The launcher library will print arguments separated by a NULL character, to allow arguments with
 # characters that would be otherwise interpreted by the shell. Read that in a while loop, populating
 # an array that will be used to exec the final command.
 CMD=()
 while IFS= read -d '' -r ARG; do
   CMD+=("$ARG")
-done < <("$RUNNER" -cp "$SPARK_ASSEMBLY_JAR" org.apache.spark.launcher.Main "$@")
+done < <("$RUNNER" -cp "$LAUNCH_CLASSPATH" org.apache.spark.launcher.Main "$@")
 
 if [ "${CMD[0]}" = "usage" ]; then
   "${CMD[@]}"
diff --git a/bin/spark-class2.cmd b/bin/spark-class2.cmd
index 4b3401d745..3d068dd3a2 100644
--- a/bin/spark-class2.cmd
+++ b/bin/spark-class2.cmd
@@ -46,13 +46,22 @@ if "%SPARK_ASSEMBLY_JAR%"=="0" (
   exit /b 1
 )
 
+set LAUNCH_CLASSPATH=%SPARK_ASSEMBLY_JAR%
+
+rem Add the launcher build dir to the classpath if requested.
+if not "x%SPARK_PREPEND_CLASSES%"=="x" (
+  set LAUNCH_CLASSPATH=%SPARK_HOME%\launcher\target\scala-%SPARK_SCALA_VERSION%\classes;%LAUNCH_CLASSPATH%
+)
+
+set _SPARK_ASSEMBLY=%SPARK_ASSEMBLY_JAR%
+
 rem Figure out where java is.
 set RUNNER=java
 if not "x%JAVA_HOME%"=="x" set RUNNER=%JAVA_HOME%\bin\java
 
 rem The launcher library prints the command to be executed in a single line suitable for being
 rem executed by the batch interpreter. So read all the output of the launcher into a variable.
-for /f "tokens=*" %%i in ('cmd /C ""%RUNNER%" -cp %SPARK_ASSEMBLY_JAR% org.apache.spark.launcher.Main %*"') do (
+for /f "tokens=*" %%i in ('cmd /C ""%RUNNER%" -cp %LAUNCH_CLASSPATH% org.apache.spark.launcher.Main %*"') do (
   set SPARK_CMD=%%i
 )
 %SPARK_CMD%
diff --git a/launcher/src/main/java/org/apache/spark/launcher/AbstractCommandBuilder.java b/launcher/src/main/java/org/apache/spark/launcher/AbstractCommandBuilder.java
index d8279145d8..b8f02b9611 100644
--- a/launcher/src/main/java/org/apache/spark/launcher/AbstractCommandBuilder.java
+++ b/launcher/src/main/java/org/apache/spark/launcher/AbstractCommandBuilder.java
@@ -186,12 +186,24 @@ abstract class AbstractCommandBuilder {
       addToClassPath(cp, String.format("%s/core/target/jars/*", sparkHome));
     }
 
-    final String assembly = AbstractCommandBuilder.class.getProtectionDomain().getCodeSource().
-	getLocation().getPath();
+    // We can't rely on the ENV_SPARK_ASSEMBLY variable to be set. Certain situations, such as
+    // when running unit tests, or user code that embeds Spark and creates a SparkContext
+    // with a local or local-cluster master, will cause this code to be called from an
+    // environment where that env variable is not guaranteed to exist.
+    //
+    // For the testing case, we rely on the test code to set and propagate the test classpath
+    // appropriately.
+    //
+    // For the user code case, we fall back to looking for the Spark assembly under SPARK_HOME.
+    // That duplicates some of the code in the shell scripts that look for the assembly, though.
+    String assembly = getenv(ENV_SPARK_ASSEMBLY);
+    if (assembly == null && isEmpty(getenv("SPARK_TESTING"))) {
+      assembly = findAssembly();
+    }
     addToClassPath(cp, assembly);
 
-    // Datanucleus jars must be included on the classpath. Datanucleus jars do not work if only 
-    // included in the uber jar as plugin.xml metadata is lost. Both sbt and maven will populate 
+    // Datanucleus jars must be included on the classpath. Datanucleus jars do not work if only
+    // included in the uber jar as plugin.xml metadata is lost. Both sbt and maven will populate
     // "lib_managed/jars/" with the datanucleus jars when Spark is built with Hive
     File libdir;
     if (new File(sparkHome, "RELEASE").isFile()) {
@@ -299,6 +311,30 @@ abstract class AbstractCommandBuilder {
     return firstNonEmpty(childEnv.get(key), System.getenv(key));
   }
 
+  private String findAssembly() {
+    String sparkHome = getSparkHome();
+    File libdir;
+    if (new File(sparkHome, "RELEASE").isFile()) {
+      libdir = new File(sparkHome, "lib");
+      checkState(libdir.isDirectory(), "Library directory '%s' does not exist.",
+          libdir.getAbsolutePath());
+    } else {
+      libdir = new File(sparkHome, String.format("assembly/target/scala-%s", getScalaVersion()));
+    }
+
+    final Pattern re = Pattern.compile("spark-assembly.*hadoop.*\\.jar");
+    FileFilter filter = new FileFilter() {
+      @Override
+      public boolean accept(File file) {
+        return file.isFile() && re.matcher(file.getName()).matches();
+      }
+    };
+    File[] assemblies = libdir.listFiles(filter);
+    checkState(assemblies != null && assemblies.length > 0, "No assemblies found in '%s'.", libdir);
+    checkState(assemblies.length == 1, "Multiple assemblies found in '%s'.", libdir);
+    return assemblies[0].getAbsolutePath();
+  }
+
   private String getConfDir() {
     String confDir = getenv("SPARK_CONF_DIR");
     return confDir != null ? confDir : join(File.separator, getSparkHome(), "conf");
diff --git a/launcher/src/main/java/org/apache/spark/launcher/CommandBuilderUtils.java b/launcher/src/main/java/org/apache/spark/launcher/CommandBuilderUtils.java
index f4ebc25bdd..8028e42ffb 100644
--- a/launcher/src/main/java/org/apache/spark/launcher/CommandBuilderUtils.java
+++ b/launcher/src/main/java/org/apache/spark/launcher/CommandBuilderUtils.java
@@ -30,6 +30,7 @@ class CommandBuilderUtils {
   static final String DEFAULT_MEM = "512m";
   static final String DEFAULT_PROPERTIES_FILE = "spark-defaults.conf";
   static final String ENV_SPARK_HOME = "SPARK_HOME";
+  static final String ENV_SPARK_ASSEMBLY = "_SPARK_ASSEMBLY";
 
   /** Returns whether the given string is null or empty. */
   static boolean isEmpty(String s) {
diff --git a/launcher/src/test/java/org/apache/spark/launcher/SparkSubmitCommandBuilderSuite.java b/launcher/src/test/java/org/apache/spark/launcher/SparkSubmitCommandBuilderSuite.java
index 626116a9e7..97043a76cc 100644
--- a/launcher/src/test/java/org/apache/spark/launcher/SparkSubmitCommandBuilderSuite.java
+++ b/launcher/src/test/java/org/apache/spark/launcher/SparkSubmitCommandBuilderSuite.java
@@ -98,7 +98,7 @@ public class SparkSubmitCommandBuilderSuite {
       parser.NAME,
       "appName");
 
-    List<String> args = new SparkSubmitCommandBuilder(sparkSubmitArgs).buildSparkSubmitArgs();
+    List<String> args = newCommandBuilder(sparkSubmitArgs).buildSparkSubmitArgs();
     List<String> expected = Arrays.asList("spark-shell", "--app-arg", "bar", "--app-switch");
     assertEquals(expected, args.subList(args.size() - expected.size(), args.size()));
   }
@@ -110,7 +110,7 @@ public class SparkSubmitCommandBuilderSuite {
       parser.MASTER + "=foo",
       parser.DEPLOY_MODE + "=bar");
 
-    List<String> cmd = new SparkSubmitCommandBuilder(sparkSubmitArgs).buildSparkSubmitArgs();
+    List<String> cmd = newCommandBuilder(sparkSubmitArgs).buildSparkSubmitArgs();
     assertEquals("org.my.Class", findArgValue(cmd, parser.CLASS));
     assertEquals("foo", findArgValue(cmd, parser.MASTER));
     assertEquals("bar", findArgValue(cmd, parser.DEPLOY_MODE));
@@ -153,7 +153,7 @@ public class SparkSubmitCommandBuilderSuite {
     String deployMode = isDriver ? "client" : "cluster";
 
     SparkSubmitCommandBuilder launcher =
-      new SparkSubmitCommandBuilder(Collections.<String>emptyList());
+      newCommandBuilder(Collections.<String>emptyList());
     launcher.childEnv.put(CommandBuilderUtils.ENV_SPARK_HOME,
       System.getProperty("spark.test.home"));
     launcher.master = "yarn";
@@ -273,10 +273,15 @@ public class SparkSubmitCommandBuilderSuite {
     return contains(needle, list.split(sep));
   }
 
-  private List<String> buildCommand(List<String> args, Map<String, String> env) throws Exception {
+  private SparkSubmitCommandBuilder newCommandBuilder(List<String> args) {
     SparkSubmitCommandBuilder builder = new SparkSubmitCommandBuilder(args);
     builder.childEnv.put(CommandBuilderUtils.ENV_SPARK_HOME, System.getProperty("spark.test.home"));
-    return builder.buildCommand(env);
+    builder.childEnv.put(CommandBuilderUtils.ENV_SPARK_ASSEMBLY, "dummy");
+    return builder;
+  }
+
+  private List<String> buildCommand(List<String> args, Map<String, String> env) throws Exception {
+    return newCommandBuilder(args).buildCommand(env);
   }
 
 }